The invention relates to a composite differential amplifier and more particularly to a composite differential amplifier which enables an output DC voltage of the differential amplifier to be constant.
The conventional composite differential amplifier, as shown in FIG. 1, comprises a differential amplifier, an emitter-follower, resistances 207 and 208 and an operational amplifier 215. The differential amplifier comprises an input signal source 214, transistors 201 and 202, resistances 205 and 206 and a constant current source 212. The emitter follower comprises transistors 203 and 204 and constant current sources 210 and 211.
An operation of the above mentioned composite differential amplifier will hereinafter fully be described in detail with reference to the accompanying drawings.
Referring to FIG. 1, a signal v.sub.i generated by the input signal source 214 is amplified by the differential amplifier. The amplified signal is fed to the base of each of the transistors 203 and 204 of the emitter-follower. The each signal is further amplified by the transistors 203 and 204 and is delivered from each of the emitter thereof. Thus, the emitter output voltage V.sub.o is given by EQU V.sub.O =V.sub.OP -R.sub.L I.sub.O -Ay.sub.i -Y.sub.Be ( 1).
where A is the gain of the differential amplifier, 2I.sub.O is the current value of the constant current source 212, R.sub.L is the resistance value of the resistance 205, V.sub.OP is the output voltage of the operational amplifier 215, V.sub.BE is the voltage between base and emitter of the transistors 203.
While an output voltage V.sub.O ' of the emitter of the transistor 204 is also given by EQU V.sub.O '=V.sub.OP -R.sub.L I.sub.O +Av.sub.i -V.sub.BE ( 2).
where A is the gain of the differential amplifier, 2I.sub.O is the current value of the constant current source 212, R.sub.L is the resistance value of the resistance 206, V.sub.OP is the output voltage of the operational amplifier 215 and V.sub.BE is the voltage between base and emitter of the transistor 204.
Since the resistance value of the resistance 207 is equal to the resistance value of the resistance 208, an inverting input voltage V.sub.i of the operational amplifier 215 is the middle-point-voltage between the emitter output voltage V.sub.O of the transistor 203 and the emitter output voltage V.sub.O ' of the transistor 204. Then, the inverting input voltage V.sub.1 is given by ##EQU1##
It may therefore be understood from the equation (4) that the inverting input voltage V.sub.i is the output DC voltage which is delivered from the differential amplifier through the emitter-follower. Then, the output voltage V.sub.OP of the operational amplifier is given by EQU A.sub.OP (V.sub.C -V.sub.i)=V.sub.OP ( 4).
where A.sub.OP is the open-gain of the operational amplifier, V.sub.C is the external control voltage of the operational amplifier 215, V.sub.i is the inverting input voltage of the operational amplifier 215.
The following equation (5) is obtained by substituting the above equation (3) in the above equation (4). EQU A.sub.OP (V.sub.C -V.sub.i)=V.sub.i +R.sub.L I.sub.O +V.sub.BE( 5).
Then, the output DC voltage from the differential amplifier is given by ##EQU2##
Assuming that the open-gain A.sub.OP of the operational amplifier 215 is considerably large, the following equation is obtained. When the gain A of the operational amplifier is considerably large, the equation (6) is made to the following equation (7): EQU V.sub.i .perspectiveto.V.sub.C ( 7).
Consequently, the output DC voltage of the differential amplifier is nearly equal to the external control voltage with independence from values of the resistance R.sub.L, the current I.sub.O and the base-emitter voltage V.sub.BE. It therefore may be assumed that the external control voltage V.sub.C may be regulated to be constant by the regulators so as to obtain the constant output DC voltage of the differential amplifier.
In the conventional composite differential amplifier, the output of the operational amplifier 215 serves as a power supply of the differential amplifier. Thus, a high output DC voltage V.sub.DC of the differential amplifier may be set with a higher output voltage V.sub.OP of the operational amplifier, whereby a source voltage of the operational amplifier is needed to be much higher than the output voltage V.sub.OP of the operational amplifier. Further, all circuit currents of the composite differential amplifier are supplied with only the output of the operational amplifier to substantially increase a power consumption of the operational amplifier with a higher external control voltage of the operational amplifier, whereby it is impossible to lower the operational amplifier voltage.